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== Plot Summary ===== Part One: Long Knives ===[[Warmaster]] [[Horus]] has fallen to [[Chaos]], but his plans of rebellion against the [[Emperor]] are known only to a few he trusts to side with him. [[Primarchs]] [[Angron]], [[Mortarion]] and [[Fulgrim]] of the [[World Eaters]], [[Death Guard]] and [[Emperor's Children]] respectively have all sided with Horus and their four Legions gather in the Isstvan system on the pretence of crushing a rebellion by the Isstvanians. The unconscious [[Euphrati Keeler]] has developed a cult following who believe the [[Emperor]] is a deity and she is a saint, [[Kyril Sindermann]] chief among them. The group gets word that forces loyal to Horus are coming questions Chief [[Astropath]] [[Ing Mae Sing]] about the saint and afterwards Sing sends a psychic warning to kill her and Sindermann, who recruits [[Titus Cassar]] of the [[Legio Mortis ]] [[Titan]] Legion begs and his reluctant friend [[Jonah Aruken]] to help him save her. Aruken grudgingly agrees and he, Cassar and Sindermann go to rescue Keeler, only to find [[Maggard]], former bodyguard of [[remembrancer ]] [[Petronella Vivar]] but now turned genetically-enhanced enforcer for Horus , about to kill her. Maggard attacks them but becomes unable to move, the bullets from his gun moving very slowly, and the three men realize that Keeler has awoken and is using her powers to save herself and them. Before Maggard can stop them they carry Keeler to safety amidst the bowels of the ''[[Vengeful Spirit]]'''s lower decks. Horus sacrifices Sing in order to facilitate an audience with [[Chaos]] as revenge for sending the warning.

The assault force that will descend to the surface of Emperor's Children and Death Guard attack an enemy station on [[Isstvan III to persecute Extremis]] where they discover a [[Warsinger]] leading the war defenders. Death Guard [[Captain]] [[Nathaniel Garro]] is chosen squad-badly wounded by-squad, which the Warsinger's sonic power but [[Lord Commander]] [[Eidolon]] is unorthodox, piquing able to kill it with his own sonic shriek. This arouses the suspicion of [[Saul Tarvitz]] of the Emperor's Children. Tarvitz talks his way into remaining in the fleet when the assault force deploys , who questions Eidolon about it and on the gun decks discovers the ship's crew loading lethal Virus Bombs into the ship's weapons. Realizing that the Space Marines deployed to the surface are about is taken to be betrayed and wiped out, Tarvitz steals a [[ThunderhawkApothecary]] and flies for the surface to warn them. He is pursued by members of his own Legion who are part of the betrayal and in desperation communicates with [[Nathaniel GarroFabius]], Battle-Captain of 's laboratory where he is shown the Death Guard 7th Great Company who many surgical enhancements Fabius is stuck in-fleet after losing making on legion members and offered a leg place in a preliminary engagement against the rebel Isstvaniansnew order. Garro, an old friend of TarvitzHorrified by Fabius'sexperiments, believes him when he tells him of Tarvitz rejects the imminent betrayal offer and orders his friagte Eisenstein to fire on the Thunderhawks pursuing Tarvitzleaves. The resulting cloud An assault force containing elements of debris hides Tarvitz from the fleet's sensors as he lands on Isstvan all four legions chosen by Horus and Garro reports that Tarvitz is dead before preparing to jump to the including [[Garviel Loken]], [[WarpTarik Torgaddon]] and bring warning of Horus's betrayal to the [[EmperorLucius]]descends on Isstvan III's capital city and battle breaks out.

Meanwhile on === Part Two: The Choral City === Suspicious at the surface those Space Marines hand-picked nature of the [[Sons of Horus]]assault force, World Eaters, Emperor's Children Tarvitz talks his way into remaining in the fleet when the assault force deploys and Death Guard whom their primarchs deemed too loyal to the Emperor to rebel against him battle investigates the rebel Isstvaniansgun decks, where he discovers the Sons of Horus and Emperorship's Children led by crew loading lethal [[Garviel Lokenvirus bombs]]into the ship's guns. Realizing that the marines deployed to the surface are about to be betrayed and wiped out, Tarvitz steals a [[Tarik TorgaddonThunderhawk]] and [[Lucius]] taking flies for the enemy strongholdsurface to warn them. Saul Tarvitz arrives and warns them He is pursued by members of their imminent betrayal and most his own legion who are part of them are able to take shelter before the Virus Bombs fall. Crewing a Titan on the surface Jonah Aruken betrayal and Titus Cassar hear in desperation puts himself at the command to power down and seal their machinemercy of [[Nathaniel Garro]], their Princeps being loyal to Horus and so having received warning. Cassar realizes what who is happening and tries to sabotage excluded from the Titan, pleading with Aruken to help him, but Aruken chooses ambition over loyalty assault force and shoots Cassar dead. In the fleet Horus gathers the remembrancers attached relegated to commanding the four Legions and shows them the bombing before ordering his men to open fire, massacring them. Kyril Sindermann and frigate ''[[Mersadie OlitonEisenstein]] escape alongside Euphrati Keeler when she leads them '' due to [[Iacton Qruze]]his injuries. Garro, an elderly Captain old friend of Tarvitz's, believes him when he tells him of the imminent betrayal and orders the Sons of Horus, who leads them away from ''Eisenstein'' to fire on the massacreThunderhawks pursuing Tarvitz. The four arrive at resulting cloud of debris hides Tarvitz from the Vengeful Spiritfleet's hangar and plan sensors as he lands on Isstvan, Garro reporting that Tarvitz is dead before preparing to jump to steal a Thunderhawk but are intercepted by Maggard. Qruze and Maggard fight but are evenly matched until Kyril Sindermann picks up a blade and stabs Maggard in the side, allowing Qruze to finish him off. The four steal a Thunderhawk [[Warp]] and flee, guided by Keeler bring warning of Horus's betrayal to the Eisenstein where Nathaniel Garro takes them aboardEmperor.

On the surface the [[Sons of Horus]], World Eaters, Emperor's Children and Death Guard whom their primarchs deemed too loyal to the Emperor to rebel against him battle the rebel Isstvanians. The Sons of Horus' [[Drop Pods|drop pods]] scatter throughout the target area and they struggle to link up and capture their objective, the Sirenhold. Lucius leads the Emperor's Children half of the attack on the Precentor's Palace while the World Eaters under Captain [[Ehrlen]] become bogged down massacring fanatical citizens in a plaza to the north and the Death Guard advance through the city's defensive trenchworks. Lucius succeeds in killing the rebel governor [[Vardus Praal]] and the Emperor's Children capture the palace. Tarvitz then arrives and warns them of their imminent betrayal, tasking Lucius with transmitting a warning to the Sons of Horus and Death Guard while he goes to warn the World Eaters. Awakened screaming by a premonition of the virus strike, Euphrati Keeler transmits the vision to Sindermann and [[Mersadie Oliton]], who realise the betrayal that is about to take place. The majority of the Emperor's Children and Sons of Horus are able to take shelter beneath the palace in time, but few of the World Eaters or Death Guard reach the shelter of bunkers and more are killed when Horus follows up the virus-bombs with an orbital strike that ignites all the gas in the atmosphere, burning the entire planet. Crewing the ''Dies Irae'' in support of the Death Guard Jonah Aruken and Titus Cassar hear the command to power down and seal their machine, their princeps [[Esau Turnet]] being loyal to Horus and so having received warning. Cassar realizes the truth after they ride out the virus strike and firestorm and attacks Turnet, pleading with Aruken to help him, but his insistence that the Emperor is a god alienates Aruken who does not believe and chooses ambition over friendship, shooting Cassar dead. In the fleet Horus gathers the remembrancers from the ''Vengeful Spirit'' and shows them the bombing before ordering his men to open fire, massacring them. Kyril Sindermann and Mersadie Oliton escape alongside Euphrati Keeler when she leads them to [[Iacton Qruze]], the Sons of Horus' elderly and often dismissed third captain, who leads them away from the massacre. The four arrive at the ''Vengeful Spirit'''s hangar and plan to steal a Thunderhawk but are intercepted by Maggard. Qruze and Maggard fight but are evenly matched until Sindermann picks up Maggard's dropped pistol and shoots him in the side, allowing Qruze to finish him off. The four steal a Thunderhawk and flee, guided by Keeler to the ''Eisenstein'' where Nathaniel Garro takes them aboard. === Part Three: Brothers === Infuriated by the survival of the loyalists on the surface , Angron spearheads a World Eaters' attack wave to destroy them without waiting for Horus's orders. Angron and his traitor World Eaters assault their loyalist counterparts and Tarvitz flees on Ehrlen's orders to take command of the other survivors. Forced to support him Angron Horus deploys the traitor Sons of Horus, Emperor's Children and Death Guard, turning what was meant to be a single wipeout blow from orbit into a protracted ground war. Saul Tarvitz emerges as a brilliant leader, commanding the loyalists as they repel wave after wave of traitor attacks while slowly being whittled down. Frustrated and jealous that Tarvitz is getting all the glory Lucius slays traitor Emperor's Children [[Chaplain]] [[Charmosian ]] and uses his helmet microphone to contact the traitorsEidolon, selling out the loyalists in exchange for a place in his traitor Legionamong the traitors. Lord Commander Horus commits the Sons of Horus under [[Serghar Targost]] and [[EidolonLuc Sedirae]] to the battle while sending [[Ezekyle Abaddon]] and [[Horus Aximand]] to confront their former [[Mournival]] brothers Loken and Torgaddon. Eidolon launches a devastating attack that hits , penetrating the rebels palace where they are vulnerablehe massacres the loyalist wounded. Receiving word of Abaddon and Aximand's arrival Loken and Torgaddon leave the palace, acting Loken being attacked by [[Kharn]] until a Sons of Horus' tank drives over them and impales Kharn on its dozer blades. Tarvitz finds Luciusamong the corpses of loyalist Emperor's informationChildren and discovers his betrayal. The pair fight, while and Tarvitz is hopelessly outmatched until he copies something Loken once did when fighting Lucius in the practice cages and drops his weapon in favour of punching Lucius in the face. A squad of loyalists arrive and open fire on Lucius slips away , but he manages to escape. Leading the surviving loyalists he pulled out of the fight, Tarvitz takes advantage of Eidolon's undisciplined attack and Garviel manages to repel the assault.  Loken and Tarik Torgaddon go to confront their traitorous former [[Mournival]] brothers [[Ezekyle Abaddon]] and [[Horus Aximand]]in the ruins of the city's parliament building. The confrontation splits into two duels as Loken fights Abaddon and Torgaddon fights Aximand. Outclassed against Abaddon Loken knows he needs Torgaddon's help to win, but Aximand kills Torgaddon, leaving Loken helpless against . Abaddon who grievously wounds him , but before he can finish Loken off the ''Dies Irae'' steps on the building and brings the roof down. Abaddon pulls Aximand from the rubble, noting his sorrow at murdering Torgaddon, and believing Loken dead they are fighting in is destroyed by a traitor Titanreturn to the fleet. The traitor forces pull back, With the rebels loyalists all but destroyed, the traitors fall back and Horus orders the fleet to firebomb firebombs the planet city from orbit, scouring it Tarvitz and the scant few survivors bracing for the end knowing that they hurt the traitors and bought time for Garro to reach Terra. Buried beneath the rubble of all remaining lifethe parliament building, the last thing that Loken sees is the bombs falling towards him.